This invention relates, in general, to a wireline communication system and an associated method of allocating frequencies for use in traffic communications. More particularly, but not exclusively, the present invention is applicable to bi-directional wireline communication systems that support digital subscriber line (xDSL) communication protocols, which wireline communication systems inherently suffer from the undesirable effects of cross-talk interference.
Telecommunication systems that interconnect wireline subscriber terminals are being developed to support broadband data communication. More particularly, recent developments in broadband communication protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometers, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometers. Line codes such as discrete multi-tone (DMT), which can be used for Very high-speed Digital Subscriber Line (VDSL), utilise multiple sub-channel carriers, e.g. in a DMT environment, to provide an adaptive system that mitigates the effects of cross-talk by selectively ignoring noise-effected sub channel carriers or reducing the number of bits supported by the sub-channels. As will be appreciated, DMT provides a comb of frequency carriers that are each separated modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
Presently, some xDSL systems (and the like) utilise a time division duplex transmission scheme in which a communication resource (such as a dedicated channel within frequency limits) has a time-split use for unlink and down-link transmissions between line termination equipment (LTE) and customer premises equipment (CPE). More specifically, the up-link and down-link may have different traffic capacities, i.e. there is a fixed but potentially asymmetric symbol capacity (or number of time slots) between the up-link and the down-link assigned for the entire duration of a call. For example, in an Internet-type environment, It is usually beneficial to have a higher down-link capacity since information download is the dominant data flow, whereas voice traffic generally requires equal traffic capabilities in both directions.
In frequency division duplex (FDD) systems, spectrum is allocated between the up-link and down-link.
In relation to bundles of wireline communication resources, it is also important to consider the potentially undesirable effects associated with cross-talk interference. Specifically, with bi-directional communication, the relative location of the lines, for example, between twisted copper-pair causes cross-talk interference to be induced into proximately located wireline communication resources (principally by the mechanisms of capacitive and inductive coupling and by radiation arising from the imperfect nature and performance of the cabling). Moreover, where symmetrical and asymmetrical service are simultaneously required on pairs in the same bundle, cross-talk becomes a significant problem, as will readily be appreciated.
For completeness, it will be understood that near-end cross-talk (NEXT) occurs when electromagnetic interference is induced into a wireline resource that is communicating information in an opposing direction, e.g. down-link (or downstream) information appears as noise in an unlink (or upstream) path. NEXT is undesirable because near-end generated interference is at a level that can potentially swamp data signals received from a remote terminal, which data signals have previously been subjected to attenuation through the transmission path. Furthermore, NEXT increases significantly at the higher frequency components and so is even more undesirable in high frequency data-over-voice wireline systems, such as VDSL. To avoid the harmful effects of Near-End Cross-Talk (NEXT) in a TDD system, an ensemble of collated communication resources must have synchronised and aligned transmissions. However, in a mixed symmetrical-asymmetrical system, NEXT often occurs where the two opposing schemes have either different frequency allocations (in frequency division duplex, FDD) or different time slot allocations (in TDD).
The skilled artisan will appreciate that the partitioning of symbols (or time slots) between up-link and down-link transmissions must also take into account the form of traffic that is to be sent in the respective directions, and this is reflected by the present schemes of fixing symbol (or time-slot) allocation for the entire duration of a call. Specifically, for voice communication (as opposed to data transmissions) it will be appreciated that the nature of verbal expression requires regular information to be conveyed in order to support a coherent understanding. In contrast, data can be of a bursty nature since a reception pattern for information in somewhat irrelevant and there can, in fact, be a reordering of information at a receiver. As such, voice communication in any telecommunication scheme generally requires the fixed provisioning of sufficient capacity/bandwidth.
With regard to Far End Cross-Talk (FEXT), this form of cross-talk affects non-addressed ports of a remote terminal in other words, FEXT occurs when electromagnetic interference (i.e. noise) is induced into a wireline resource that is communicating information in a similar direction, e.g. upstream (or up-link) information appears as noise in another upstream wireline resource to an extent that performance on a given pair is limited. The effects of FEXT are correspondingly reduced by the attenuation path of the wireline resource However, when multiple separate modem links exist (as supported by a multiplicity of different copper pairs proximally located towards the exchange LTE as a bundle of pairs in the access network and then fanned out to individual drops serving particular CPEs), crosstalk between the numerous signals at (or towards) the exchange presently generates noise that limits the data-rate performance of both a given pair and the entire wireline system, in general. In synchronised systems, FEXT is inherent.
FEXT on adjacent pairs can be severely exacerbated from increased signal strengths at a receiver modem of the exchange LTE for the pair causing the FEXT. More especially, where these adjacent pairs have shorter reaches (i.e. shorter cable lengths), the attenuation of the signal in the wireline resource from such relatively closely located modems (as opposed to remotely located modems) is relatively little and, correspondingly, FEXT induced into adjacent wireline resources can be relatively large. In other words, in instances when FEXT from a relatively closely located modem is introduced into a wireline resource serving a distantly located modem, the FEXT interference effects can be catastrophic and corruption of the data from the distant modem absolute. For this reason, it is accepted that there is a need to xe2x80x9cback-offxe2x80x9d the power transmitted by the transmitting (CPE) modem in the upstream direction for all but the longest lines. Unfortunately, the necessity for back-off results in inefficient utilisation of the spectrum as a consequence of CPEs served by short loop distances having to forego the benefits of better signal to noise ratios (SNR) and therefore to restrict channel throughput by reducing power and lowering the bit transmission rate. In summary, FEXT is particularly problematic in the up-link at the LTE and limits spectral capacity generally.
The term xe2x80x9cself-FEXTxe2x80x9d will be understood to mean FEXT arising from use of the same time-slot and/or the same frequency for a common form of service (as opposed to differing services on a common wireline resource, such as a combination of ADSL and VDSL).
With respect to power back-off of a particular copper pair, existing methods propose the measurement of loss (i.e. attenuation) at a single frequency and the subsequent projection of loss for the entire bandwidth for that particular copper pair, and the corresponding determination of upstream power (PSD). This method if unreliable because the power transmission characteristics of a copper pair vary according to frequency and, in the extreme, the selected frequency (carrier) can coincide with resonance, for example, within the wireline resource to an extent that a misleading indication of the general transmission properties of the wireline resource is derived. Additionally, such single frequency techniques utilise a centralised decision-making process for all wireline resources incident to the exchange that limits the performance of all loops to be the same as (i.e. as poor as) the performance associated with the longest reach. Power back off can be achieved on either an equal margin basis or an equal power basis. Equal margin allows for a fixed excess in SNR terms, for example, with respect to all incident lines at the receiver, namely the LTE for upstream communications. In contrast, equal power ensures that the received power at the LTE is the same for all lines at any, several, or all frequencies. In both cases, the power spectral density of the transmitted signal is varied to take into account the variable noise floor and the variable level of attenuation associated with the selected carrier frequency and the loop length.
In an attempt to address the problems associated with cross-talk in baseband line coded systems, some manufacturers have also resorted to echo-cancellation techniques. Unfortunately, while echo cancellation compensates a particular twisted copper-pair for its return duplex path, echo cancellation does not mitigate or even address the effects caused by cross-talk induced by other distinct systems, i.e. other twisted copper-pairs. Furthermore, echo cancellation techniques are generally expensive.
Wireline service must also preserve certain (or sensitive) frequency bands, and in this respect some proposed regulations already require the radio amateur spectral bands (at least) to be notched out (i.e. filtered) to avoid carrier interference through a radiation process from the wireline communication resource. This concept of selective notching is already prevalent in most radio-based communication systems, such as within the Digitally Enhanced Cordless Telecommunications (DECT) system and the Global System for Mobile (GSM) communication, and is reflected in electromagnetic compatibility (EMC) requirements.
Furthermore, as will also be widely appreciated, broadband access schemes require a channel response that satisfies Nyquist""s criteria, namely that sampling is required at twice the frequency of the highest information carrier, i.e. sampling occurs at 2f whilst transmission is limited to f and below. The alias-band associated with a particular system actually contains a duplication of the information contained in the baseband.
It will therefore be appreciated that present systems are generally intolerant of the variation in attenuation for different length loops (i.e. lines) and have a performance that is limited to the lowest performance of a copper pair in a bundle this being particularly so in relation to up-link transmissions that are subject to the effects of FEXT.
According to a first aspect of the present invention there is provided a method of allocating frequency bandwidth in a wireline communication network maintaining a plurality of wireline links between a line termination node containing a plurality of collocated modems and a plurality of subscriber-associated modems located at differing loop lengths from the line termination node, the method comprising he steps of estimating a loop length of at least one wireline link; and partitioning bandwidth on a frequency basis based on estimations of the loop length of the at least one wireline link.
In a preferred embodiment, the step of partitioning bandwidth further comprises the steps of: assigning at least some of the plurality of subscriber-associated modems to a relatively low frequency range bandwidth and limiting a signal strength for all frequency carriers in an upstream direction in the relatively low frequency range bandwidth to a signal strength substantially corresponding to a signal strength received in the upstream direction in relation to a subscriber-associated modem having a longest loop length in the relatively low frequency range bandwidth; and assigning at least one of the plurality of subscriber-associated modems to a relatively high frequency range bandwidth and limiting a signal strength for all frequency carriers in an upstream direction in the relatively high frequency range bandwidth to a signal strength substantially corresponding to a signal strength received by a modem in the line termination node in relation to a subscriber-associated modem having a longest loop length in the relatively high frequency range bandwidth.
The subscriber-associated modems assigned to the relatively low frequency range bandwidth may be excluded, if desired, from the relatively high frequency range bandwidth.
The relatively low frequency range bandwidth is distinguished from the relatively high frequency range bandwidth by a threshold frequency at which the received signal strength in the upstream from a subscriber having the longest loop length in the relatively low frequency range bandwidth becomes one of: substantially indistinguishable from a noise floor of the system; and inadequate for supporting useful data transmissions.
In another embodiment of the present invention there is provided a method of partitioning bandwidth comprises the step of: assigning base-band to a first group of subscriber-associated modems; and assigning alias-band frequencies to a second group of subscriber-associated modems having shorter loop lengths than at least some of those subscriber-associated modems in the first group.
The second group of subscriber-associated modems may contain modems requiring a reduced channel capacity.
In a second aspect of the present invention there is provided a wireline communication system comprising line termination equipment coupled to a plurality of modems via wireline resources of varying loop length, the wireline resource having means for providing a frequency partitioned bandwidth for supporting a plurality of upstream transmission from the plurality of modems to the line termination equipment, and wherein the bandwidth is partitioned based on loop length basis.
Preferably, the second bandwidth includes the first bandwidth and a further portion of bandwidth above the first threshold frequency, and wherein the second communication has a power spectral density limited so as to provide the predetermined signal strength level to the line termination equipment in the first bandwidth and a different power spectral density providing a different received signal strength in the second bandwidth.
In another embodiment the means for providing a frequency partitioned bandwidth further comprises: complementary switchable filters in the line termination equipment and the plurality of modems, the switchable filters arranged to select between base-band and alias-band: means for assigning base-band to a first group of modems; and means for assigning alias-band frequencies to a second group of modems having shorter loop lengths than at least some of those modems in the first group.
The second group of subscriber-associated modems contains modems generally require a reduced channel capacity.
In another aspect of the present invention there is provided a modem-management system combination comprising means for providing a frequency partitioned bandwidth based on a loop length of a wireline resource.
The numerous aspects of the present invention therefore provide an improved performance for FEXT-limited communication systems through the segregation of frequency carriers based on loop length. Put another way, the present invention provides partitioning of a larger proportion of the available spectrum based on loop length, since the power spectral density of channel resources assigned to CPEs located relatively close to an LTE can be increased to support higher bit rates by virtue of the assigned communication resources, including spectrum being isolated in frequency from those channel resources used by CPEs remotely located from the LTE. In this way, overall system performance is improved and cross-talk reduced on a general basis within the system. Indeed, the cross-talk environment can be reduced by virtue of the fact that, provided sufficient capacity exists within the higher frequency bands. GPEs relatively closely located to the CPE (in terms of loop length) can benefit from the exclusive use of higher channel frequencies to an extent whereby lower channel frequencies may be used solely to service relatively distantly located GPEs (i.e. those CPEs having a long loop length).
Having regard to another inventive notion, there is provided a method of allocating frequency bandwidth for communication in a wireline system supporting transmissions between a plurality of subscribers and a central node, said transmissions limited to a predetermined maximum power level for the wireline system and wherein the wireline system has a theoretical interference floor, the method comprising: having regard to the theoretical interference floor, identifying a maximum frequency at which reliable communication is deemed occur between a selected subscriber at a first distance and the central node and transmitting information at the predetermined maximum power level; attenuating the predetermined maximum power level for transmissions at said maximum frequency between at least a second subscriber at a second distance and the central node when the second distance is shorter than the first distance, thereby to limit interference: in relation to a second frequency higher than said maximum frequency at the first distance, transmitting information at the predetermined maximum power level; and at frequencies intermediate between the first frequency and the second frequency and for subscribers at nominally constant distance from the central node increasing transmit power levels towards the predetermined maximum power level.
In a preferred embodiment a level of increase in the transmit power for the intermediate frequencies is determined as an attenuation of the predetermined maximum power level, the attenuation calculated as the loss arising from a cable at a frequency in question for a length difference between a length in question and a length whose maximum frequency is the frequency in question.
Preferably, the method of the further inventive notion further comprises the step of, in relation to subscriber at nominally constant distance from the central node, transmitting at the predetermined maximum power level for frequencies above the second frequency.